1. Field of the Invention
This invention relates to sheet handling systems. It has particular applicability to sheet handling systems for use in fused image printers and copiers.
2. Description of Related Developments
In a transfer electrostatographic process such as conventional transfer xerography, in which an image pattern of dry particulate unfused toner material is transferred to a final image support surface, e.g., a copy sheet from an initial image bearing surface, e.g., a charged photoreceptor surface developed with toner, the transferred toner is typically only loosely applied to the final image support surface after transfer, and is easily disturbed by the process of stripping the final image support surface away from the initial support surface and by the process of transporting the final image support surface to the toner fusing station. The final image support surface preferably passes through a fusing station as soon as possible after transfer to fuse the toner image permanently onto the final image support surface, thereby preventing smearing or disturbance of the toner image by mechanical agitation or electrical fields. For this reason, and also for the reasons of simplifying and shortening the paper path of the copier, it is desirable to maintain the fusing station as close as possible to the transfer station. A particularly desirable fusing station is a roll-type fuser, wherein the copy sheet is passed through a pressure nip between two rollers, at least one of which is heated and at least one of which is resilient.
However, when such a fuser roll nip for the final image support surface is located close enough to the transfer station so that a lead portion of the final image support surface can be in the fuser roll nip simultaneously with the rear or trailing portion of the same final image support surface still being in contact with the photoreceptor, smears or skips in the unfused toner image, which is being transferred to the trailing portion of the final image support surface, can occur. This condition is caused by relative movement or slippage between the initial support surface and the final image support surface in those areas where they are still in contact, i.e., those areas of the final image support surface which has not yet been stripped away from the initial support surface. A source of such slippage is a speed mismatch between the nip speed of the fuser rolls (the speed at which the fuser is pulling the lead edge of the paper through the fuser) relative to the surface speed of the initial support surface. If the fuser nip roll is slower, the final image support can slip backwards relative to the initial image support surface. If the fuser roll is faster, the final image support material can be pulled forward relative to the image on the initial support surface. In either case, this can cause the aforementioned smears or skips in the toner image to be transferred to the trailing area of the final image support or to cause image elongation.
An exactly equal velocity drive connection between the initial support surface and the fuser rolls is difficult to maintain. Also, there is a further complication that the actual sheet driving velocity of the fuser nip roll can change with changes in an effective diameter of the driving roll in the nip. This can occur from replacement of the rollers or changes in the resilient deformation of the rollers due to changes in applied nip pressure, material aging, temperature effects, etc. Thus, equal speed is difficult to maintain between a fuser nip roll and the photoreceptor surface in commercial printing apparatus and can require increased maintenance and the need for speed adjustment mechanisms.
In order to overcome these problems, three basic design approaches have been taken. The first is to allow enough paper path distance between transfer and fusing to accommodate most paper sizes with minimum disturbance to unfused toner particles. This solution has the effect of increasing the length of the paper path, thereby requiring the copier to occupy a large floor area. This is disadvantageous, especially to customers having limited space availability or having high floor space costs.
A second approach is to use complex paper paths with special transports. This solution is undesirable because it adds cost to the equipment and introduces potential sources of maintenance requirements and unreliablity.
A third approach is to use buckle chambers between the transfer station and the fuser so that speed mismatches between the transfer station and the fuser rolls can be accommodated by the portion of the image support surface that is in the buckle. U.S. Pat. No. 4,017,065 shows one such buckle arrangement. In the designs disclosed in this patent, the image surface is formed in a buckle by being drawn, by vacuum, against a guide surface. The fuser roll nip is intentionally driven at a different speed than the transfer speed to form a buckle. The buckle is controlled by cyclic reductions in the vacuum applied to the guide surface. Another approach is shown in U.S. Pat. No. 4,941,021, wherein a buckle is formed by controlling the speed of the fuser rolls so that the image support surface travels more slowly through the fuser rolls than through the transfer zone. This system requires sensing of the buckle to maintain the size of the buckle within predetermined limits. Such sensing systems add manufacturing cost and require maintenance, as dust and dirt within the equipment can interfere with sensing, particularly when optical detectors are used.